Arel compatibility

fixes #6 

(not really pretty - but shows a possible fix)
Maybe we could avoid this branching (and thus the double definition of the `matches` and `does_not_match` methods) by using the splat operator.
Since we don't actually use the `escape` argument within the methods, and `super` called without parens forwards the arguments as is, I guess this would do the trick for all versions:
```ruby
def matches(other, *)
  if Regexp === other
    matches_regexp(other)
  else
    SexyScopes.extend_predicate(super)
  end
end
```
I'd be grateful if you could give it a try.